1. Field of the Invention
This invention pertains to the manufacture of semi-fluid lubricants.
2. Description of the Prior Art
Semi-fluid lubricants such as traction motor gear lubricants are essentially mineral oils thickened by a low percentage of soap. The percentage of soap normally used in a finished product is between 2 and 3 percent. Traction motor gear lubricants are used, for example, in the diesel-electric locomotive. In such a locomotive, power is transmitted from the electric motor through a set of spur gears, a pinion gear on the motor shaft and a large ring gear attached to the axle of the locomotive. The entire gear arrangement is usually enclosed in a gear case which also serves as a lubricants reservoir. The lubricant in the case is maintained at a level to partially submerge the ring gear. Consequently, the lubricant must have a viscosity so that it is fluid enough to seek its own level in the bottom of the case, but adhesive enough to both cling to the gears and resist leakage from the case. Therefore, recently users of these traction motor gear lubricants have related performance of the lubricants to the apparent viscosity and not the penetration value as was formerly the case.
The conventional method of controlling the consistency of traction motor gear lubricants was to manufacture to a specific worked penetration (method ASTM D217). However, viscosity tests results on traction motor gear lubricants which were manufactured to a specified worked penetration value were consistently far above the specified viscosity requirements. In a specific application where a viscosity of 5,000 to 15,000 CP was required, only five of 14 batches or 36% complied, immediately after manufacture but before packaging. Seven percent exceeded the 15,000 CP maximum limit and eight batches were below the 5,000 CP limit before packaging. However, after packaging, during which the lubricants were subjected to additional shear, viscosities ranged from 39,000 to 50,000 CP, far above the acceptable limits imposed by the new viscosity requirement.
These high apparent viscosity values of the traction motor gear lubricants were related to shear thickening experienced during handling subsequent to manufacture including the packaging operation which consisted of filling polyethylene bags.
Generally during the polyethylene bag filling operation, it is necessary to maintain the temperature of the lubricant at about 130.degree.. The desired uniform temperature is maintained in the tank car by using a gear pump to continuously recirculate the lubricant through an outside line from the bottom of the tank into the top of the tank car. The temperature of the lubricant is maintained in the tank car by instrument controlled internal heating coils. A line leading from the circulation loop fed the bagging machine. For satisfactory function of the bagging operation, it is necessary to maintain a bag pressure of approximately 85 psig on the circulation loop. It is evident that the traction gear lubricants manufactured according to classical procedure shear thickened excessively during the packaging operation and could not be packaged to the proposed apparent viscosity requirements.
In the prior art method, a concentrated soap base containing about 5 to 6 percent soap with the balance being oil was shear thickened and then diluted to attain a required final soap content, usually between 2 and 3 percent. Since traction gear lubricants manufactured by this method shear thickened to excessive apparent viscosities, it became necessary to develop a manufacturing method whereby the penetration value of the finished lubricant would not be altered and yet the viscosity would fall within the range of specifications. This also required that the finished semi-fluid lubricant contained the same 2 to 3 percent soap concentration.